Steam engines which operate in accordance with the well-known Rankine cycle have excellent greenhouse gas emission characteristics compared to internal combustion engines and accept a wide range of liquid and solid fuels including even organic waste and biomass which makes steam power an especially attractive alternative to engines requiring refined petroleum. Moreover, a catalytic converter is not necessary to meet emissions standards since steam generator combustion temperature at atmospheric pressure is below that required to create compounds of NOx. However, in the past, steam engines in smaller sizes suitable for use in cars and trucks have been inefficient compared to the internal combustion engine. A journal article Expander Efficiency by Stan Jakuba, Steam Automobile Club of America Bulletin, Vol. 14, No. 2, April-June 2000, giving the results of 21 Rankine development programs, listed steam engines with a service life of over two years as having an Actual Overall Cycle Efficiency that ranged from 6% to 17.2%, of which the best three averaged 14.4%. This level of efficiency is insufficient to be competitive with combustion engines for vehicular use.
Although it has been reported in the literature that a reduced clearance space between the piston at the top center position and the cylinder head will improve efficiency in a steam engine, a significant clearance volume has been present in the past. First, the steam inlet and outlet passages add a certain volume. In addition, in actual engines it has been traditionally considered important to provide a degree of compression at the end of each return stroke to achieve a cushioning effect, in other words, a steam cushion to help balance the reciprocating forces. For example, in U.S. Pat. No. 863,545, one object listed is to reduce clearance space. However, due to the cavities shown in the patent for allowing valve motion as well as what is referred to as a steam cross passage and head opening, a sizable clearance volume is unavoidable. Moreover, the need for two steam cam shafts with gearing adds to the cost and bulk of the engine.